Process for recovering dimethylnaphthalenes from cracked gas-oil by azeotropic distillation



3,171,794 5 FROM March 1965 R. WYNKOOP ET AL PROCESS FOR RECOVERING DIMETHYLNAPHTHALENE CRACKED GAS-OIL. BY AZEOTROPIC DISTILLATION Filed May '24, 1961 w m m TK m v A m 293mm o u o 0mm Em DG 2 time ..r I NH 2 5.36 626 5- 5 mw 3. 224 m. 9.2 2220 26: mw RJ n. m mm o.

o. M .8903 30 6 cm 022200 *Qm zo muw E282 33B. $30k :256 .3.5.220 Cm oowm. o m m .m m. m. 2 39.334 1 2255.52.35 em re? $03 03 33m 25:20 =o 8w 22 motuEcaoz ATTORNEY United States Patent PROCESS FOR RECOVERING DIMETHYLNAPH- THALENES FROM CRACKED GAS-OIL BY AZEO- TROPIC DISTILLATION Raymond Wynkoop, Gladwyne, and Joseph G. Allen, Ridley Park, Pa., assignors to Sun Oil Company, Philadelphia, Pa., a corporation of New Jersey Filed May 24, 1961, Ser. No. 112,466 13 Claims. (Cl. 20242) This invention relates to a process for segregating petroleum hydrocarbon mixtures into relatively aromatic and relatively non-aromatic fractions. It particularly relates to a method for separating from such mixtures dimethylnaphthalene concentrates. It especially relates to a process for recovering 2,6- and 2,7-dimethylnaphthalene from cracked gas-oil by azeotropic distillation with diethylene glycol.

The synthetic fiber market has achieved significant importance in the commercial world. One of the more important materials from which such fibers are made is the polyester of 2,6-dicarboxynaphthalene. Therefore, it is an object of this invention to produce an aromatic concentrate which is rich in 2,6-dimethylnaphthalene which is an intermediate for preparing the diester monomer. Specifically, the principal object of this invention is to recover from cracked gas-oil an aromatic concentrate composed almost entirely of 2,6- and 2,7-dimethylnaphthalene. The 2,6-isomer can be separated from 2,7-isomer by, say, fractional crystallization. The process of this invention is economical and simple and it produces av dimethylnaphthalene concentrate from which the 2,6- isomer can be obtained in high purity.

The present invention is based on the discovery that under narrow conditions of temperature and solvent dosage the desired, say, 2,6- and 2,7-DMNs will form an azeotrope with a solvent such as diethylene glycol. Therefore, the present invention provides a process for recovering these isomers from a suitable charge material via simple azeotropic distillation."

The charge material used in the present invention is obtained as a cracked gas-oil fraction. boiling in the range of 400 F. to 650 F. which contains alkyl naphthalene and dialkyl naphthalene constituents. Within this definition of starting material are included cracked gas-oils as obtained directly from distillation of cracked products, such gas-oils having an initial boiling point not substantially lower than 400 F. and a final boiling point not substantially higher than 650 vR; mixed aromatic concentrates obtained. from such cracked gas-oils and having a boiling range substantially the same as the starting cracked gas-oil; and fractions separated from either of the above materials but which boil within the narrower range of 475 F. to 520 F., preferably between 480 F. and 515 F. As used herein the term cracked includes thermal, catalytic, and reforming operations.

According to the present invention, the cracked petroleum fraction should contain substantial amounts of dicyclic aromatic compounds and preferably little if any tricyclic aromatic compounds. Since the petroleum fraction can be derived from practically any source of crude petroleum, its specific composition can vary considerably. Even though the boiling range of the petroleum fraction can be between 400 F. and 650 F., it is preferred that the boiling range be substantially between 475 F. and 520 F., more preferably between 480 F. and 515 F., so that a concentrate of such dicyclic compounds as dimethylnaphthalene (DMN) can be obtained. Furthermore, according to this invention, the charge material in contact with the solvent, described hereinafter, should 3,171,794 Patented Mar. 2, 1965 have total aromatic content of at least 40% by weight and preferably 60% by weight. In fact, best results are obtained if the charge material is aromatic hydrocarbons. Further, the cracked fraction should contain at least 23% by weight DMNs, preferably from 5% to 60% by weight DMN.

Additionally, suitable charge materials within the definition of cracked gas-oils will have an API gravity at 60 F. between 12 and 40; a refractive index at 20 C. of from 1.4500 to 1.5800; and a sulfur content of from 0.05% by weight to 3.0% by weight. Best results are obtained, however, if the charge material contains less than 0.2% sulfur. Stocks having higher sulfur contents preferably are desulfurized prior to practicing the process of the invention.

The solvents utilized in the present process are those which are selective for aromatics and which are selective azeotrope formers with the dialkyl naphthalenes. Suitable solvents are the polyalkylene glycols, including, for example, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, tripropylene glycol, and mixed ethylene glycol-propylene glycol ethers. The preferred solvent or azeotroping agent is diethylene glycol (DEG).

The selectivity of the solvent can be enhanced to a considerable extent by the addition thereto of small amounts of water. For example, certain limited amounts of water, say, up to 20%, and preferably up to 10% by weight, of the solvent composition will enhance the selectivity of the solvent between dicyclic aromatics and non-aromatic hydrocarbons. In any event, it has been found that at least 0.25% water is needed to effectively cause the dimethylnaphthalenes to azeotrope with diethylene glycol.

Thus, the present invention provides a process for recovering dimethylnaphthalene concentrate from cracked gas-oil which comprises contacting cracked gas-oil containing dimethylnaphthalenes and boiling substantially within the range of 400-650 F. preferably 475 F. 520 F. with a solvent which is selective for aromatics and which is a selective azeotrope former with dialkyl naphthalenes; subjecting the solvent-oil mixture to azeo tropic distillation; removing an overhead fraction consisting essentially of dimethylnaphthalenes and solvent; recovering the dimethylnaphthalenes from said solvent; and removing a bottoms fraction as a residuum.

The invention is more specifically described with reference to the accompanying flow diagram which is a schematic illustration of the present process.

The feedstock, hereinbefore described, is charged through line 10 into a first distillation tower 11. An overhead fraction comprising light gas-oil boiling between about 400 F. and 480 F. is removed through line 12. A first bottoms fraction comprising heavy gas-oil boiling higher than 515 F. is removed via line 13. The desired cracked gas-oil fraction boiling between 480-515 F. is passed as a distillate fraction from tower 11 through line 14 wherein at point 15 it is admixed with diethylene glycol solvent which enters via line 16. The solvent dosage should be at least 3 parts of DEG per 2 parts hydrocarbon. An excess amount of DEG is desirable. Suitable solvent/hydrocarbon ratios ordinarily vary between about 1 to 10 but depending upon the feedstock character istics this factor must be varied in order to azeotrope only the dimethylnapththalenes. The preferred range of solvent/ feed ratio is 1.5 to 2.5.

The solvent-feed mixture enters a second distillation tower 17 from which non-aromatic and other hydrocarbons distilling below about 450 F. are removed via line 18. The heavier aromatic fraction comprising essentially concentrated dimethylnaphthalenes plus the solvent are passed as a second bottomsfraction from tower 17 through line 19 into azeotropic distillation tower 20. The dimethylnaphthalene-DEG azeotrope which distills beginning at about 451 F. and ending at about 454 F. is removed via line 21. Suitable reflux is returned throu h line 23 to control the reflux ratio between about 2 and 30. Prefcrabl'y the reflux ratio will be from about 321 to 52:1. The composition of the DEG-DMN a'zeotr'ope in line 24 will be about 58% by 'weight DEG and 42% by weight DMN. The residuum aromatic hydrocarbons distilling above about 455 F. are removed through line 22. In all cases, the DEG can be recovered and recycled to the process to effect economies in the operation.

The azeotropic distillation is ordinarily conducted at atmospheric pressures, but elevated or reduced pressures may be employed if desired.

The DEG-DMN .azeotrope removed through line 24 can be-further processed, such as by cooling to less than 100 F., in order to break the azeotrope into two phases. The DMN is recovered by, say, decantation, from the DEG and the recovered solvent is usually recycled to the process. p

The concentrate of 2,6- and 2,7-dimethylnaphthal'ene can be further processed. For example, 2,6-DMN of 90+% purity can be recovered from the 2,7-DMN via fractional crystallization.

Quantitatively the above process can be described as follows: 7 Charge: 100 parts cracked gas-oil boiling between 400 F. and 650 F. I 1st distillation step (tower 11):

PrOducts- Parts Lt. gas-oil (400480 F.) 12 Med. gas-oil (4805l5 F.) 24 Hy. gas-oil (5l5-650 F.) 64

2nd distillation step (tower 17):

Charge: 24 parts med. gas-oil, 48 parts DEG containing 2% water.

Products- Non-aromatic and other hy- 17 parts drocarbons overhead--- (450 F. max). DEG overhead; 30 parts. DMN overhead 1 part. C12 aromatic bottoms 6 parts. DEG bottoms 18 parts.

Azeotropic distillation step (tower 20) 7 Charge: 6 parts C aromatics, 18 parts DEG Products- Overhead DMNs 5 parts (451-454 F). DEG overhead 7 parts. Bottom hydrocarbons 1 part. DEG bottoms 11 parts;

Wt. Percent Azeotrope Distillation Wt. Percent Total Aromatics in 480'515 I". Cut

were cent DMN Wt. Percent DMN in init.-448 1. Cut in 448460 F. Cut

Therefore, from the above data it is concluded that at least 40% total aromatics are needed in the feed material, preferably at least 60% aromatics, before a significant concentration of dimethylnaphthalenes appears in the azeotrope. Further, it was discovered that the dimethylnaphthalene concentrate will contain an equilibrium mixture of the DMN isomers if the azeo-trope is removed over a temperature range of 448 F. to 460 F. On the other hand, it'was discovered that the 2,6- and 2,7-isomers are the major product distilling at 452 F. but are essentially absent from the product distilling above 454 F Thus, the present invention provides a process for recovering an aromatic concentrate composed of 2,6-

and 2,7-dimethylnaphthalenes from catalytic gas-oil containing said dimethylnaphthalenes and boiling between 400 F.650 P. which comprises processing said gasoil to obtain a hydrocarbon fraction within the range of 480 F.-5l5 F. and containing at least 40% aromatic hydrocarbons; contacting said fraction with aqueous diethylene glycol solvent; subjecting said solvent-hydrocarbon mixture to fractionation; removing overhead the hydrocarbons distilling below 450 F.; removing a bottoms fraction; subjecting the bottoms fraction to azeotropic distillation; removing overhead a hydrocarbonsolvent mixture a'zeotroping between 451 F. and 454 F.; cooling said azeotropic mixture to break the mixture into a solvent phase and a 2,6- and 2,7-dimethylnaphthalene phase; recovering the 2,6- and 2,7-dimethylnaphthalenes from the solvent; and removing from the distillation zone those aromatic hydrocarbons distilling above 454 F.

We claim:

1. A process for recovering an aromatic concentrate composed of 2,6- and 2,7-dimethylnaphthalenes from cracked gas-oil containing 26- and 2,7-dimethylnaphthalenes which comprises contacting cracked gas-oil containing dirnethylnaphthalenes and boiling substantially within the range of 475-520 F. witha polyalkylene glycol solvent which is selective for aromatics and which is a selective azeotrop'e former for dialkyl naphthalenes; sub- 'jecting the solvent-oil mixture to'azeotropic distillation; removing 'an overhead fraction consisting essentially of dimethylnaphthalenes and solvent; recovering 2,6- and 2,7-di'methylnaphthalenes from said solvent; and removing a bottornsfraction as a residuum.

2. A process according to claim 1 wherein said solvent is diethylene glycol.

3. A process according to claim 2 wherein said solvent contains at least 0.25% by weight Water.

4. A processaccording to claim 1 wherein said overhead fraction is removed at a temperature between 448 F. and 460 F.

5. A process according to claim 4 wherein said solvent is diethylene glycol. I

6. A process for recovering an aromatic concentrate composed of 2,6- and 2,7-dimethylnaphthalenes from catalytic gas-oil which comprises processing said gas-oil to obtain a hydrocarbon fraction boiling within the range of 480 F. to 515 F. and containing at least 40% aromatic hydrocarbons; contacting said fraction with aqueous diethylene glycol solvent; subjecting "said solventhydrocarbon mixture to fractionation; removing overhead the hydrocarbons distilling belo'w 450 F.; removing a bottoms fraction; subjecting the bottoms fraction'to azeotropic distillation; removing overhead a hydrocarbonsolvent mixture azeotroping between 451 F. and 454 F.; cooling said azeotropic mixture to break the mixture into a solvent phase and a 2,.6- and 2,7-dimethylnaphthalene 'phase; recoveringthe 2,6- and 2,7-dimethylnaph'thalenes nonrure'solvent; and removing from the distillation zone those aromatic hydrocarbons distilling above 454 'F. 7. A-process according to claim 6 wherein said hydrocarbon fraction contains at least 60% aromatic hydrocarbons. V

8. A process according to claim 7 wherein said diethylene' glycol contains from 0.25% to 20% water.

9. A process according to claim 6 wherein said diethylene glycol contains from 0.25% to 10% water.

10. Process for recovering an aromatic concentrate composed of 2,6- and 2,7-dimethylnaphthalenes from a cracked gas-oil feed stock boiling within the range of 400650 F. and containing dimethylnaphthalenes which comprises:

(1) passing said feed stock into a first distillation zone,

(a) removing overhead a light gas-oil stream boiling within the range of 400-480 F.,

(b) removing a first bottoms fraction boiling higher than 515 F.,

(c) removing a distillate fraction boiling between (2) admixing said distillate fraction with a polyalkylene glycol solvent which is selective for aromatics and which is a selective azeotrope former with dimethylnaphthalenes,

(3) passing said admixtureinto a second distillation zone,

(a) removing overhead a stream comprising nonaromatic hydrocarbons,

(b) removing a second bottoms fraction comprising essentially dimethylnaphthalenes,

(4) subjecting said second bottoms fraction to azeotropic distillation,

(a) removing overhead a dimethylnaphthalenesolvent azeotrope distilling within a temperature range of 448 F. to 460 F.

(b) removing a residue fraction comprising residuum aromatic hydrocarbons, and,

(5) recovering 2,6- and 2,7-dimethylnaphthalenes from said azeotrope.

11. Process according to claim 10 wherein said solvent is aqueous diethylene glycol.

12. Process according to claim 11 wherein said dimethylnaphthalene solvent azeotrope distills over a temperature range of 451 to 454 F.

13. Process according to claim 12 wherein said distillate fraction contains at least 60 percent aromatic hydrocarbons.

References Cited in the file of this patent UNITED STATES PATENTS 2,265,939 Field Dec. 9, 1941 2,411,106 Petry et a1. Nov. 12, 1946 2,878,261 Broughton Mar. 17, 1959 2,909,576 Fenske et al Oct. 20, 1959 FOREIGN PATENTS 668,853 Great Britain Mar. 26, 1952 1,037,756 France Sept. 22, 1953 

1. A PROCESS FOR RECOVERING AN AROMATIC CONCENTRATE COMPOSED OF 2,6- AND 2,7-DIMETHYLNAPHTHALENES FROM CRACKED GAS-OIL CONTAINING 2,6- AND 2,7-DIMETHYLNAPHTHALENES WHICH COMPRISES CONTACTING CRACKED GAS-OIL CONTAINING DIMETHYLNAPHTHALENES AND BOILING SUBSTANTIALLY WITHIN THE RANGE OF 475-520*F. WITH A POLYALKYLENE GLYCOL SOLVENT WHICH IS SELECTIVE FOR AROMATICS AND WHICH IS A SELECTIVE AZEOTROPE FORMER FOR AROMATICS AND WHICH IS A SELECTED AZEOTROPE FORMER FOR DIALKYL NAPHTHALENES; SUBJECTING THE SOLVENT-OIL MIXTURE TO AZEOTROPIC DISTILLATION; REMOVING AN OVERHEAD FRACTION CONSISTING ESSENTIALLY OF DIMETHYLNAPHTHALENES AND SOLVENT; RECOVERING 2,6- AND 2,7-DIMETHYLNAPHTHALENES FROM SAID SOLVENT; AND REMOVING A BOTTOMS FRACTION AS A RESIDUUM. 